1. Field of the Invention
The present invention relates to a method for transferring a toner image formed electrostatically on the surface of a photoconductive plate or a dielectric material onto the surface of a sheet of toner receiving material in an electrophotographic copying apparatus or an electrostatic printer, and a transfer device for use in practicing the said method.
2. Prior Art
Heretofore, as a device for transferring a toner image formed electrostatically on the surface of a photoconductive plate or a dielectric material onto the surface of a sheet of toner receiving material (hereinafter reffered to as a sheet of paper) in an electrophotographic copying apparatus, there has widely been used a charging device which utilizes corona discharge such as corotron. In this type of charging device, a fine metallic wire such as tungsten wire which is difficult to be damaged by corona discharge, or such metallic wire coated with an insulating material such as glass, is used. In using such charging devices for the transfer purpose, a high DC or AC voltage of 4,000 to 8,000 volts is applied between the aforementioned metallic wire (discharge wire) and a sheath-like metallic case called a shield which surrounds the discharge wire in spaced relation, thereby inducing corona discharge around the discharge wire, and the resultant positive or negative electric charge is released from an opening portion of the above shield and applied to the back of a sheet of paper, thereby inducing a transfer field between the paper and the photosensitive plate or dielectric material. Consequently, a high voltage current source is needed for inducing corona discharge around the discharge wire. Moreover, for the stabilization of discharge, the greater part of the corona discharge current is allowed to flow the shield without direct utilization for the transfer.
Further, because of the necessity of applying an electric charge uniformly to the paper, the fine metallic wire must be spaced 1 cm or so from the surface of a photosensitive plate or a dielectric material. Consequently, the electric charge is applied more widely than necessary to the back of the transfer part of the paper and the transfer field spreads, so that the toner image to be transferred is apt to be disturbed. This tendency is marked particularly when the photosensitive plate is a drum surface of a small diameter. In applying an electric charge to the paper, the applied charge may leak and cause a poor transfer in the case where the paper is influenced by the ambient humidity and its resistance is thereby decreased. To avoid this inconvenience, it is necessary to take measures for facilitating transfer of a toner image, for example, electrically insulating a sheet of paper under transfer from surrounding objects, or weakening the electrostatic adhesion between the toner image and the photosensitive plate or dielectric material.
In separating the paper from the photosensitive plate or dielectric material after transfer, the paper adheres to the surface of the photosensitive plate or dielectric material due to the electric charge applied thereto, and is therefore difficult to separate, thus requiring means for deelectrifying the paper after transfer. Moreover, due to the inflow of corona ion to the surface of the photosensitive plate, the photosensitive plate is apt to be deteriorated. Further, from the characteristic standpoint, it is difficult to form the section of the charging device not larger than several square centimeters.
As transfer means, a roller type transfer device is known in addition to the above charging device which utilizes corona discharge. In this transfer device, a semiconductive roller or a conductive roller having an insulating layer on the surface thereof, with DC voltage applied thereto, is pushed against a toner image formed on the surface of a photosensitive plate or the like to thereby effect transfer of the toner image. In this transfer device, the amount of electric charge applied to a sheet of paper is less than that in the charging device using the corona discharge, and the applied voltage can be somewhat reduced. But, due to pressing, the transferred toner image is apt to undergo transfer defects, e.g. image omission. Further, the roller type transfer device involves the problem of increase in size of the apparatus.
Thus, the main transfer methods presently in use have various inconveniences.